SnS2 Nanoparticles and Thin Film for Application as an Adsorbent and Photovoltaic Buffer

Energy consumption and environmental pollution are major issues faced by the world. The present study introduces a single solution using SnS2 for these two major global problems. SnS2 nanoparticles and thin films were explored as an adsorbent to remove organic toxic materials (Rhodamine B (RhB)) from water and an alternative to the toxic cadmium sulfide (CdS) buffer for thin-film solar cells, respectively. Primary characterization tools such as X-ray photoelectron spectroscopy (XPS), Raman, X-ray diffraction (XRD), and UV-Vis-NIR spectroscopy were used to analyze the SnS2 nanoparticles and thin films. At a reaction time of 180 min, 0.4 g/L of SnS2 nanoparticles showed the highest adsorption capacity of 85% for RhB (10 ppm), indicating that SnS2 is an appropriate adsorbent. The fabricated Cu(In,Ga)Se2 (CIGS) device with SnS2 as a buffer showed a conversion efficiency (~5.1%) close to that (~7.5%) of a device fabricated with the conventional CdS buffer, suggesting that SnS2 has potential as an alternative buffer.


Introduction
Global energy consumption is anticipated to increase dramatically over the next few decades. This is primarily due to the expected global population expansion as well as the economic and industrial growth of developing nations, among other factors. According to a new report from the International Energy Agency (IEA), worldwide energy consumption has declined by approximately 1% in 2020 because of the COVID-19 pandemic and is expected to increase by approximately 5% in the coming years [1]. Sustainable energy sources are growing rapidly, but not fast enough to meet the significant increase in global energy demand, resulting in a dramatic increase in coal consumption that threatens to increase carbon dioxide emissions from the energy industry to historic levels. Electricity generation from PV technology is more affordable than that from non-renewable fossil fuel sources. As a result, it is necessary to advance the field of photovoltaic (PV) research by utilizing sustainable materials.
Notably, solar technologies based on Cu(In, Ga)Se 2 (CIGS) thin-film semiconducting materials are available at a price comparable to or lower than that of standard silicon modules [2]. The typical structure of CIGS thin film technology comprises a bottom contact layer made of molybdenum (Mo), a p-type CIGS absorber layer (1-3 µm), a thin n-type cadmium sulfide (CdS) buffer layer, a zinc oxide (ZnO)-based transparent window layer [3], and top metal contacts. Generally, conventional CdS buffer is produced by chemical bath deposition (CBD), which shields the absorber layer from direct current (DC) sputtering damage and alters the surface of the CIGS absorber [4]. In addition, the chemical bath that contains dyes. Indeed, most dyes are chemically stable for their intended purposes and are challenging to break down biologically [17]. As a result, photocatalysis [18,19], chemical oxidation [20], and adsorption [21] are the primary methods for removing dyes from aqueous solutions. Adsorption is viable for removing hazardous dyes from aqueous solutions as it is simple to conduct, inexpensive, environmentally benign, efficient, and requires little energy [22].
To address the two distinct types of challenges faced by the world, the authors suggested a single material as the solution. Tin disulfide (SnS 2 ) is a benign, affordable, and simple binary IV-VI group metal chalcogenide with a band gap of~2.9 eV and an n-type conductivity. It consists of earth abundant elements with easily controllable chemical stoichiometry and high chemical and environmental stability. It showed a peculiar CdI 2 -type layered structure, with tin atoms sandwiched between two layers of hexagonally arranged close-packed sulfur atoms [23]. It forms an Ohmic contact with metals without current losses. In addition, excellent structural flexibility, broader spectral response, and better thermal stability of SnS 2 make it a competitive non-toxic substitute for various applications, such as solar cells, field effect transistors, thin film diodes, high-speed photodetectors, lithium-sodium ion batteries, supercapacitors, catalysts, and gas sensors [24][25][26][27][28]. Because of its appropriate band gap, it transmits most of the solar radiation to the absorber and minimizes the parasitic absorption loss. It has carrier density of the order of 10 19 cm −3 and electron affinity of 4.1 eV [29] to achieve sufficient type-inversion within the absorber surface and to make a favorable conduction band offset (CBO, 0-0.4 eV) with the absorber, respectively [30]. Further, it is chemically stable in both acidic and neutral aqueous solutions [21]. Therefore, SnS 2 has the potential to be a promising non-toxic buffer in solar cells [29,31] and an effective adsorbent for the degradation of organic pollutants [32].
In this paper, we present the synthesis of SnS 2 nanoparticles using a facile chemical precipitation approach. Subsequently, SnS 2 thin films were formed by spin coating the SnS 2 nanoparticles. Further, the SnS 2 nanoparticles were investigated as a possible adsorbent for removing a toxic dye, Rhodamine B (RhB), from an aqueous solution. In addition, SnS 2 thin films were explored as a buffer layer for CIGS solar cells.

Synthesis of SnS 2 Nanoparticles for Adsorbent Application
The procedure for the synthesis of SnS 2 nanoparticles is shown in Figure 1. A simple chemical precipitation method was used to synthesize SnS 2 nanoparticles. For a typical synthesis, 0.1 M of stannous chloride, 0.8 M of thioacetamide, and 1 M of trisodium citrate were aggressively stirred on the magnetic hot plate. The solution was allowed to react for 50 min at a constant temperature of 60 • C with a solution pH of 2. The SnS 2 nanoparticles were collected and cleaned using deionized water before drying in a vacuum oven at 70 • C for 4 h. Finally, the as-synthesized SnS 2 nanoparticles exhibited a golden yellow hue. The SnS 2 nanoparticles/powder was collected in a vial and desiccated for further analyses.
SnS2 nanoparticles. Further, the SnS2 nanoparticles were investigated as a possible adsorbent for removing a toxic dye, Rhodamine B (RhB), from an aqueous solution. In addition, SnS2 thin films were explored as a buffer layer for CIGS solar cells.

Synthesis of SnS2 Nanoparticles for Adsorbent Application
The procedure for the synthesis of SnS2 nanoparticles is shown in Figure 1. A simple chemical precipitation method was used to synthesize SnS2 nanoparticles. For a typical synthesis, 0.1 M of stannous chloride, 0.8 M of thioacetamide, and 1 M of trisodium citrate were aggressively stirred on the magnetic hot plate. The solution was allowed to react for 50 min at a constant temperature of 60 °C with a solution pH of 2. The SnS2 nanoparticles were collected and cleaned using deionized water before drying in a vacuum oven at 70 °C for 4 h. Finally, the as-synthesized SnS2 nanoparticles exhibited a golden yellow hue. The SnS2 nanoparticles/powder was collected in a vial and desiccated for further analyses.

Preparation of SnS2 Thin Film for Photovoltaic Application
The as-synthesized SnS2 nanoparticles were used for the preparation of SnS2 thin films by spin coating. SnS2 thin film deposition was accomplished by sonicating the SnS2 nanoparticles dispersed in 2 mL ethanol. Then, the resultant ink was spread out over a pre-cleaned glass substrate and a Mo/CIGS absorber. The preparation process of the SnS2 thin-film is shown in Figure 2. The SnS2 film deposited on a glass substrate was used to analyze the structural and optical properties, and the film on the Mo/CIGS absorber was utilized as a buffer during the construction of a CIGS solar cell for device analysis. The experiments for the synthesis of SnS2 nanoparticles and thin films were repeated, and both of the experimental results showed good reproducibility.

Preparation of SnS 2 Thin Film for Photovoltaic Application
The as-synthesized SnS 2 nanoparticles were used for the preparation of SnS 2 thin films by spin coating. SnS 2 thin film deposition was accomplished by sonicating the SnS 2 nanoparticles dispersed in 2 mL ethanol. Then, the resultant ink was spread out over a pre-cleaned glass substrate and a Mo/CIGS absorber. The preparation process of the SnS 2 thin-film is shown in Figure 2. The SnS 2 film deposited on a glass substrate was used to analyze the structural and optical properties, and the film on the Mo/CIGS absorber was utilized as a buffer during the construction of a CIGS solar cell for device analysis. The experiments for the synthesis of SnS 2 nanoparticles and thin films were repeated, and both of the experimental results showed good reproducibility.

Characterization Details
Characterization of SnS2 nanoparticles/thin films was done using the following techniques. The phase and elemental purity of the SnS2 nanoparticles were analyzed using a Raman spectrometer (Jobin-Yvon Lab Ram HR 800, Horiba, Kyoto, Japan) and X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Fisher Scientific, Altrincham, UK), respectively. The structural, morphological and optical properties of the SnS2 thin films were studied using a Seifert 3003TT X-ray diffractometer (Almelo, The Netherlands) with CuKα radiation (λ = 1.5405 Å), scanning electron microscope (SEM; Hitachi S-4800, Tokyo, Japan) and UV-Vis-NIR spectrophotometer (Cary 5000, Agilent, Santa Clara, CA, USA), respectively. The changes in the absorbance spectra of RhB in aqueous solution in the presence of SnS2 nanoparticles were recorded on a UV-Vis absorption spectrophotometer (Hitachi U-3010, Tokyo, Japan). The photovoltaic performance of the CIGS device with SnS2 thin film as a buffer layer was studied using illumination current density, and the voltage (J-V) characteristics were evaluated using a Solar Simulator (McScience K201 LAB50, Suwon, Korea) under AM 1.5 and 100 mW/cm 2 illumination, where the light intensity was calibrated using an Si standard reference cell.

Growth of SnS2 Nanoparticles
The following growth process is hypothesized for the synthesis of SnS2 nanoparticles. The chelating agent trisodium citrate complexes the Sn 4+ ions from stannous chloride in the first stage. The following stage involves the release of S 2− ions into the bath as a result of thioacetamide hydrolysis. The precipitation interaction between Sn 4+ and S 2− leads to the production of SnS2 nanoparticles in the final stage. The following equations describe the growth stages of the entire process: H2S + H2O ⇌ H3O + + HS − The phase purity of the prepared nanoparticles was confirmed using Raman

Characterization Details
Characterization of SnS 2 nanoparticles/thin films was done using the following techniques. The phase and elemental purity of the SnS 2 nanoparticles were analyzed using a Raman spectrometer (Jobin-Yvon Lab Ram HR 800, Horiba, Kyoto, Japan) and X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Fisher Scientific, Altrincham, UK), respectively. The structural, morphological and optical properties of the SnS 2 thin films were studied using a Seifert 3003TT X-ray diffractometer (Almelo, The Netherlands) with CuKα radiation (λ = 1.5405 Å), scanning electron microscope (SEM; Hitachi S-4800, Tokyo, Japan) and UV-Vis-NIR spectrophotometer (Cary 5000, Agilent, Santa Clara, CA, USA), respectively. The changes in the absorbance spectra of RhB in aqueous solution in the presence of SnS 2 nanoparticles were recorded on a UV-Vis absorption spectrophotometer (Hitachi U-3010, Tokyo, Japan). The photovoltaic performance of the CIGS device with SnS 2 thin film as a buffer layer was studied using illumination current density, and the voltage (J-V) characteristics were evaluated using a Solar Simulator (McScience K201 LAB50, Suwon, Korea) under AM 1.5 and 100 mW/cm 2 illumination, where the light intensity was calibrated using an Si standard reference cell.

Results and Discussion
3.1. SnS 2 Nanoparticles for Adsorbent Application 3.1.1. Growth of SnS 2 Nanoparticles The following growth process is hypothesized for the synthesis of SnS 2 nanoparticles. The chelating agent trisodium citrate complexes the Sn 4+ ions from stannous chloride in the first stage. The following stage involves the release of S 2− ions into the bath as a result of thioacetamide hydrolysis. The precipitation interaction between Sn 4+ and S 2− leads to the production of SnS 2 nanoparticles in the final stage. The following equations describe the growth stages of the entire process: The phase purity of the prepared nanoparticles was confirmed using Raman spectroscopy. The Raman spectrum of the as-synthesized nanoparticles in the region of 100-400 cm −1 is shown in Figure 3. The spectrum reveals that the as-prepared SnS 2 nanoparticles exhibited a broad peak at 310 cm −1 , which corresponds to the Raman mode associated with the hexagonal structure of SnS 2 . The phonon mode at 310 cm −1 is attributed to the vertical plane vibration (A 1g ) of the Sn-S bonds [31]. The detected Raman phonon mode was consistent with that of the previous reports [29]. 100-400 cm −1 is shown in Figure 3. The spectrum reveals that the as-prepared SnS2 nanoparticles exhibited a broad peak at 310 cm −1 , which corresponds to the Raman mode associated with the hexagonal structure of SnS2. The phonon mode at 310 cm −1 is attributed to the vertical plane vibration (A1g) of the Sn-S bonds [31]. The detected Raman phonon mode was consistent with that of the previous reports [29].

Elemental Purity Confirmation of SnS2 Nanoparticles
XPS was used to characterize the composition of the prepared nanoparticles. No significant peaks representing the contaminants were found in the spectra, suggesting that the impurity level was less than the XPS resolution limit (1 at %). Figure 4a shows a typical high-resolution spectrum of the as-synthesized SnS2 nanoparticles with binding energy ranging from 0 to 1000 eV. The wide spectrum revealed numerous peaks, such as S 2s, double S 2p, Sn 3s, doublet Sn 3p, doublet Sn 3d, and Sn 4d, indicating that the synthesized nanoparticles contain Sn and S elements. The peaks linked to C and O were possibly caused by ambient pollution. No other peaks corresponding to other elemental impurities were found in the spectrum. Figure 4b,c show the narrow XPS scans of the Sn 3d and S 2p peaks, respectively. A core-level scan of the Sn 3d doublet revealed two peaks with binding energies of 486.3 eV and 494.6 eV, corresponding to the Sn 3d5/2 and Sn 3d3/2 energy levels of Sn atoms in the Sn 4+ valence state, respectively. The peak splitting energy of Sn 3d5/2 and Sn 3d3/2 levels was approximately 8.3 eV. The binding energies of Sn 3d doublets and their spin energy separations were similar to the values reported [33]. A complete spectrum of the S 2p core level revealed two peaks with binding energies of 162.5 eV and 161.2 eV, which correspond to the S 2p5/2 and S 2p3/2 energy levels of S atoms in the Sn 2− valence state, respectively, which is consistent with the energies of the S 2− -Sn 4+ bond, confirming the presence of a single-phase SnS2 in the synthesized nanoparticles [34].

Elemental Purity Confirmation of SnS 2 Nanoparticles
XPS was used to characterize the composition of the prepared nanoparticles. No significant peaks representing the contaminants were found in the spectra, suggesting that the impurity level was less than the XPS resolution limit (1 at %). Figure 4a shows a typical high-resolution spectrum of the as-synthesized SnS 2 nanoparticles with binding energy ranging from 0 to 1000 eV. The wide spectrum revealed numerous peaks, such as S 2s, double S 2p, Sn 3s, doublet Sn 3p, doublet Sn 3d, and Sn 4d, indicating that the synthesized nanoparticles contain Sn and S elements. The peaks linked to C and O were possibly caused by ambient pollution. No other peaks corresponding to other elemental impurities were found in the spectrum. Figure 4b,c show the narrow XPS scans of the Sn 3d and S 2p peaks, respectively. A core-level scan of the Sn 3d doublet revealed two peaks with binding energies of 486.3 eV and 494.6 eV, corresponding to the Sn 3d 5/2 and Sn 3d 3/2 energy levels of Sn atoms in the Sn 4+ valence state, respectively. The peak splitting energy of Sn 3d 5/2 and Sn 3d 3/2 levels was approximately 8.3 eV. The binding energies of Sn 3d doublets and their spin energy separations were similar to the values reported [33]. A complete spectrum of the S 2p core level revealed two peaks with binding energies of 162.5 eV and 161.2 eV, which correspond to the S 2p 5/2 and S 2p 3/2 energy levels of S atoms in the Sn 2− valence state, respectively, which is consistent with the energies of the S 2− -Sn 4+ bond, confirming the presence of a single-phase SnS 2 in the synthesized nanoparticles [34]. Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 14 The adsorption of RhB dye from an aqueous solution was carried out at room temperature (approximately 25 °C). Typically, 0.4 g of SnS2 nanoparticles was added to a 1 L aqueous solution containing 10 ppm of RhB. The absorbance of RhB was monitored using UV-Vis spectroscopy for 30-180 min. The absorption spectrum of the RhB solution containing SnS2 nanoparticles is shown in Figure 5a. The absorbance spectra indicated here were taken for few drops of RhB solution after being centrifuged at a high speed (10,000 RPM for 15 min). So, the SnS2 nanoparticles in the sample are resting at the bottom of the sample, meaning there is no peak (at ~427 nm) related to SnS2. The intensity of the absorption band at 552 nm decreased linearly with the increase in time, showing that the RhB solution gradually degraded over the SnS2 adsorbent. The absorption intensity of the peak decreased by approximately 35% in the presence of SnS2 nanoparticles at 30 min. After 60 min, the degree of decolorization was 78%. After 90 min, the degree of

SnS 2 Nanoparticles as Adsorbent for Organic Pollutants (RhB Dye)
The adsorption of RhB dye from an aqueous solution was carried out at room temperature (approximately 25 • C). Typically, 0.4 g of SnS 2 nanoparticles was added to a 1 L aqueous solution containing 10 ppm of RhB. The absorbance of RhB was monitored using UV-Vis spectroscopy for 30-180 min. The absorption spectrum of the RhB solution containing SnS 2 nanoparticles is shown in Figure 5a. The absorbance spectra indicated here were taken for few drops of RhB solution after being centrifuged at a high speed (10,000 RPM for 15 min). So, the SnS 2 nanoparticles in the sample are resting at the bottom of the sample, meaning there is no peak (at~427 nm) related to SnS 2 . The intensity of the absorption band at 552 nm decreased linearly with the increase in time, showing that the RhB solution gradually degraded over the SnS 2 adsorbent. The absorption intensity of the peak decreased by approximately 35% in the presence of SnS 2 nanoparticles at 30 min. After 60 min, the degree of decolorization was 78%. After 90 min, the degree of decolorization increased to 90%, and then at 180 min, it reached 96%, suggesting that the majority of the RhB had been degraded. Discoloration of the solution might occur as a result of the dye chromogen being destroyed. Similar degradation of RhB with the increasing Zn doping concentration in SnS 2 nanoparticles was observed in a previous report [35].  The degradation of the RhB solution agrees with the pseudo-first-order kinetics [36,37], ln(C/C0) = −kt, where C0 and C are the initial and actual concentrations of RhB, k is the rate constant, and t is the degradation time. The normalized concentration of the solution equals the normalized maximum absorbance; therefore, the remaining ratio C/C0 is replaced with A/A0. The C/C0 with respect to reaction time obtained by monitoring the RhB absorption peak at 552 nm is shown in Figure 5b. It is clear that, during the initial 30 min, the concentration of RhB decreased by 65%. Further, the concentration of RhB decreased by 80% and 85% at 120 min and 180 min, respectively. Therefore, within 180 min of reaction time, SnS2 nanoparticles showed the highest adsorption capacity (21.25 mg/g) for RhB, indicating that it is an appropriate adsorbent for the removal of RhB.
The adsorption capacity of SnS2 nanoparticles was compared to that of other reported nanoparticles and nanocomposites in Table 1, which demonstrated that the adsorption performance of the SnS2 nanoparticles synthesized in the present work was similar to that of other nanocomposites and superior to that of other NP adsorbents.  The degradation of the RhB solution agrees with the pseudo-first-order kinetics [36,37], ln(C/C 0 ) = −kt, where C 0 and C are the initial and actual concentrations of RhB, k is the rate constant, and t is the degradation time. The normalized concentration of the solution equals the normalized maximum absorbance; therefore, the remaining ratio C/C 0 is replaced with A/A 0 . The C/C 0 with respect to reaction time obtained by monitoring the RhB absorption peak at 552 nm is shown in Figure 5b. It is clear that, during the initial 30 min, the concentration of RhB decreased by 65%. Further, the concentration of RhB decreased by 80% and 85% at 120 min and 180 min, respectively. Therefore, within 180 min of reaction time, SnS 2 nanoparticles showed the highest adsorption capacity (21.25 mg/g) for RhB, indicating that it is an appropriate adsorbent for the removal of RhB.
The adsorption capacity of SnS 2 nanoparticles was compared to that of other reported nanoparticles and nanocomposites in Table 1, which demonstrated that the adsorption performance of the SnS 2 nanoparticles synthesized in the present work was similar to that of other nanocomposites and superior to that of other NP adsorbents.

SnS 2 Thin Films for Photovoltaic Application
The results obtained in Section 3.1 concluded that the synthesized SnS 2 nanoparticles have a high degree of composition and phase purity. Therefore, the as-synthesized SnS 2 nanoparticles were spin coated to obtain the SnS 2 thin film. The as-synthesized SnS 2 nanoparticles showed a granular morphology with the particle size of~52 nm and the asprepared SnS 2 thin film on CIGS showed a tiny grain texture with uniformity, as displayed in Figure 6. The structural and optical properties of the SnS 2 films are described in the following sections.

SnS2 Thin Films for Photovoltaic Application
The results obtained in Section 3.1 concluded that the synthesized SnS2 nanoparticles have a high degree of composition and phase purity. Therefore, the as-synthesized SnS2 nanoparticles were spin coated to obtain the SnS2 thin film. The as-synthesized SnS2 nanoparticles showed a granular morphology with the particle size of ~52 nm and the asprepared SnS2 thin film on CIGS showed a tiny grain texture with uniformity, as displayed in Figure 6. The structural and optical properties of the SnS2 films are described in the following sections.

Crystal Structure of SnS2 Thin Film
X-ray diffraction was used to determine the crystal structure of the as-grown SnS2 film, and the matching profile is shown in Figure 7. The diffraction pattern indicated that the SnS2 film was polycrystalline, with significant peaks at 15.02°, 28.18°, 32.54°, and 49.69° corresponding to the (0 0 1), (1 0 0), (1 0 1), and (1 1 0) planes, respectively, and exhibited a hexagonal crystal structure [20]. The SnS2 peaks observed in the XRD profile are consistent with those found in the standard JCPDS card No. 23-0677. The film lacked any peaks associated with other secondary phases such as SnS and Sn2S3, suggesting that the SnS2 film generated in this work is free of contaminants [45].

Crystal Structure of SnS 2 Thin Film
X-ray diffraction was used to determine the crystal structure of the as-grown SnS 2 film, and the matching profile is shown in Figure 7. The diffraction pattern indicated that the SnS 2 film was polycrystalline, with significant peaks at 15.02 • , 28.18 • , 32.54 • , and 49.69 • corresponding to the (0 0 1), (1 0 0), (1 0 1), and (1 1 0) planes, respectively, and exhibited a hexagonal crystal structure [20]. The SnS 2 peaks observed in the XRD profile are consistent with those found in the standard JCPDS card No. 23-0677. The film lacked any peaks associated with other secondary phases such as SnS and Sn 2 S 3 , suggesting that the SnS 2 film generated in this work is free of contaminants [45].  Figure 8a shows the optical transmittance spectrum of the as-grown SnS2 film recorded in the wavelength range of 300-1000 nm. A sharp decrease in the transmittance spectrum revealed that the as-grown SnS2 film showed an average optical transmittance  Figure 8a shows the optical transmittance spectrum of the as-grown SnS 2 film recorded in the wavelength range of 300-1000 nm. A sharp decrease in the transmittance spectrum revealed that the as-grown SnS 2 film showed an average optical transmittance of approximately 90% above the fundamental absorption edge. The average transmittance difference between CdS and SnS 2 films in the visible light region is around 11%, which is a small discrepancy that could be attributed to scattering losses. The steep absorption edge in the spectrum indicates that the film has a direct optical transition between the parabolic bands. To evaluate the energy band gap (E g ) of the thin film, (αhυ) p was plotted against the photon energy (hυ), where p is an integer, whose value indicates the type of optical transition occurring in the thin film. In the present study, the nature of the transition was found to be direct with p = 1/2, and the energy band gap was calculated using plots of (αhυ) 2 versus hυ (Figure 8b), where the extrapolation of the (αhυ) 2 plot onto the hυ axis provides the band gap. The evaluated optical energy band gap of the as-prepared SnS 2 film was approximately 2.8 eV, which is consistent with previously reported findings [23]. Figure 8a,b show the transmittance spectra and corresponding band gap graph of the conventional CdS buffer. Although the transmittance of the standard CdS buffer was similar to that of the SnS 2 film, the absorption edge of the CdS film was at a longer wavelength than that of the SnS 2 film. The band gap determined from the (αhυ) 2 versus hυ plot of CdS was approximately 2.2 eV [46], which is marginally less than the band gap of the SnS 2 film. In other words, SnS 2 has a wider band gap than the conventional CdS buffer, which enables the transmission of a more significant fraction of the solar spectrum to the absorber.

SnS2 Thin Film as a Buffer in CIGS Solar Cell
In this study, two CIGS thin-film solar cells were fabricated with structures as glass/Mo/CIGS/CdS (conventional buffer)/i-ZnO/AZO/Ag/Ni and glass/Mo/CIGS/SnS2 (buffer from the present work)/i-ZnO/AZO/Ag-Ni using conventional CdS buffer and SnS2 buffer, respectively. Our earlier article [47] detailed the procedure for fabricating a CIGS absorber. Initially, a two-stage technique was used to construct a CIGS light absorber on an Mo substrate. Subsequently, the chemical bath deposition (CBD) method produced a 70 nm thick CdS buffer. The complete deposition conditions for CBD-grown CdS are described in a previous report [48]. A 50 nm thick layer of SnS2 buffer was spin-coated onto the CIGS absorber following the procedure reported in Section 2.2. The devices were then finished by radio frequency (RF)/DC sputtering of i-ZnO/ZnO/Al transparent conducting layers and e-beam evaporation of an Ni/Al grid.
The J-V characteristics of the CIGS devices made with conventional CdS buffer (CIGSCdS) and SnS2 buffer (CIGSSnS2) are shown in Figure 9. The CIGSCdS exhibited an efficiency of 7.5% with an open-circuit voltage (VOC) of 0.51 V, a short circuit current

SnS 2 Thin Film as a Buffer in CIGS Solar Cell
In this study, two CIGS thin-film solar cells were fabricated with structures as glass/Mo /CIGS/CdS (conventional buffer)/i-ZnO/AZO/Ag/Ni and glass/Mo/CIGS/SnS 2 (buffer from the present work)/i-ZnO/AZO/Ag-Ni using conventional CdS buffer and SnS 2 buffer, respectively. Our earlier article [47] detailed the procedure for fabricating a CIGS absorber. Initially, a two-stage technique was used to construct a CIGS light absorber on an Mo substrate. Subsequently, the chemical bath deposition (CBD) method produced a 70 nm thick CdS buffer. The complete deposition conditions for CBD-grown CdS are described in a previous report [48]. A 50 nm thick layer of SnS 2 buffer was spin-coated onto the CIGS absorber following the procedure reported in Section 2.2. The devices were then finished by radio frequency (RF)/DC sputtering of i-ZnO/ZnO/Al transparent conducting layers and e-beam evaporation of an Ni/Al grid.
The J-V characteristics of the CIGS devices made with conventional CdS buffer (CIGS CdS ) and SnS 2 buffer (CIGS SnS2 ) are shown in Figure 9. The CIGS CdS exhibited an efficiency of 7.5% with an open-circuit voltage (V OC ) of 0.51 V, a short circuit current density (J SC ) of 27 mAcm −2 , and a fill factor (FF) of 54%, while the CIGS SnS2 showed an efficiency of 5.1% with a V OC of 0.41 V, J SC of 26 mA cm −2 , and FF of 49%. The performance parameters of CIGS SnS2 and CIGS CdS are summarized in Table 2. The results indicated that the CIGS SnS2 device with the SnS 2 buffer prepared in this study exhibited comparable results to that of the CIGS CdS . However, the CIGS SnS2 device exhibited a considerable drop in shunt resistance (R sh ), suggesting the direct contact between the window layer and absorber possibly raised from plasma damage during window layer preparation and shunt paths caused by pinholes in SnS 2 buffer [49]. As FF is very sensitive to R sh among the device parameters [50], it is reduced by approximately 5% in the case of the CIGS SnS2 device. On the other hand, the effect of R sh is less considerable on the J SC , which is mainly governed by optical and recombination losses. The optical losses are caused by the reflection from the device surface and absorption by the AZO window and SnS 2 buffer layers. Electron-hole pairs are generated when photons hit the CIGS absorber, and some of the produced charge carriers are not collected, but are lost as a result of recombination. The recombination loss is indicative of the quality of the SnS 2 /CIGS junction (measure of V OC ) and is most sensitive to buffer thickness [49]. The plasma damage of SnS 2 and pinholes may create the AZO/CIGS interface as strong recombination centers. This kind of recombination loss may decrease with the increasing SnS 2 thickness. According to a previous report [33], the increase in the thickness of the CdS buffer layer resulted in increased light absorption loss by CdS, but decreased reflection loss. Therefore, SnS 2 thickness optimization is necessary to minimize the photocurrent loss by recombination to ensure the quality of the CIGS/SnS 2 junction and to improve the performance characteristics of the CIGS SnS2 device. increased light absorption loss by CdS, but decreased reflection loss. Therefore, SnS2 thickness optimization is necessary to minimize the photocurrent loss by recombination to ensure the quality of the CIGS/SnS2 junction and to improve the performance characteristics of the CIGSSnS2 device.

Conclusions
Tin disulfide (SnS 2 ), a simple binary metal chalcogenide, was proposed as a viable adsorbent for removing toxic dyes from water and as a buffer for Cd-free thin-film solar cells owing to its abundance, low-cost, non-toxicity, and chemical stability. The current study explored the synthesis of SnS 2 nanoparticles and the deposition of SnS 2 thin films using a chemical precipitation method and spin-coating technique, respectively. The XPS and Raman studies indicated the existence of Sn and S in the synthesized nanoparticles together with a pure SnS 2 phase (characteristic Raman mode at 310 cm −1 ). The XRD and optical studies showed that the as-synthesized SnS 2 thin films had a hexagonal crystal structure with (001) as the preferred orientation and an optical band gap of 2.8 eV. At 180 min of reaction time, 0.4 g/L of SnS 2 nanoparticles demonstrated 96% decolorization and 85% adsorption capacity for RhB (10 ppm), revealing that the majority of the RhB was degraded; therefore, SnS 2 is a suitable adsorbent. The fabricated CIGS device with SnS 2 (50 nm) as buffer exhibited an open circuit voltage (V OC ) of 0.41 V, a short circuit current density (J SC ) of 25.67 mA cm −2 , a fill factor (FF) of 49%, and a conversion efficiency (η) of 5.1%, demonstrating that SnS 2 is an alternative buffer. The authors intend to dope the nanoparticles and optimize the thickness of the SnS 2 buffer layer in future studies to (i) increase the adsorption capacity of SnS 2 nanoparticles and (ii) improve the performance of the CIGS SnS2 device.